Single cylinder deadbolt lock mechanisms that include a cylinder lock at one side thereof and a turn piece at the opposite side are, of course, well known. Conventional locks of this type provide for extending and retracting the latch bolt by use of either the lock cylinder or the turn piece. A recognized disadvantage of such lock mechanisms is that a burglar or other such unauthorized person who gains entry to a building may easily exit the same through the door after simply unlocking the same from the inside by use of the turn piece. For doors with windows, entry to the building is readily gained after first breaking the window then unlocking the door from the inside thereof. Although double cylinder deadbolt locks often are used to prevent such easy exit from a building, they are more expensive than single cylinder locks and, in the case of fire or other emergency, may prevent quick egress from the building. In some cases, local ordinances prohibit the use thereof at the primary entrance to the dwelling.
Additionally, single cylinder deadbolt lock mechanisms are known wherein the turn piece is rendered inoperative when the latch bolt is moved into locking position by key operation of the lock cylinder. Such a lock mechanism is shown in U.S. Pat. No. 2,178,666 issued to O. C. Larson. As with the above-mentioned prior art single cylinder locks of this general type, a major disadvantage of such an arrangement is that the door can not be opened from the inside using the turn piece after being locked by use of the key from the outside. Any authorized person in the building could not exit through the door once the door is locked by operation of the lock cylinder. An arrangement which avoids this disadvantage is shown in U.S. Pat. No. 4,047,408 issued to Eddie D. Johns, et al. wherein the turn piece is selectively rendered inoperable when the cylinder lock is operated by the key. With this arrangement, if the key is depressed while locking the door, the turn piece is rendered inoperative and, if not depressed, the turn piece remains operative. Major disadvantages of such an arrangement include the complexity and cost of manufacturing the same which greatly limit the marketability thereof. Also, the turn piece disabling mechanism is operated from the cylinder side of the lock rather than from the turn piece side thereby contributing to the complexity of the prior art mechanisms. In U.S. Pat. No. 4,290,282 issued to the present inventor, the turn piece is selectively rendered inoperable when the cylinder lock is operated by the key by means of a disabling mechanism that is operated from the turn piece side of the door. Again, if the turn piece has been disabled, operation of the latch bolt by the turn piece is prevented until the lock is unlatched by operation of the key.